In the co-pending application of Cuneo et al., Application No. 07/532,329, there is disclosed apparatus and a method for constructing a novel double-hulled product, which as panels, modules and midbodies, are useful in the construction of vessels, in particular, bulk carriers for crude oil and other products.
The present invention relates to improvements in the method and poducts disclosed in the above-identified, earlier application, the contents of which are incorporated herein by reference.
In general, the invention relates to providing a double-hulled vessel which, compared with conventional constructions, is made with a reduced number of different pieces, a reduced complexity, which can be fabricated using a higher degree of automation, which, in many applications is more durable and/or needs less maintenance, and need not cost the 20 percent additional that a conventional double hull costs compared with a conventional single hull. In fact, in some instances, a double hull produced in accordance with the invention can successfully compete in price with a conventional single hull for the same duty and carrying capacity.
Compared with the apparatus, method and products disclosed in the above-mentioned, earlier patent application of Cuneo et al., the present invention teaches a different method for forming the curved plates of the hull, for placing the stiffeners on the flat plates, for assembling the plates into product components for cleaning the plates prior to protectively coating the components of the product, and for protectively coating (i.e., "painting") the components.
As a result of a 1970's convention entered into by the major maritime shipping nations (the "MARPOL Convention"), bulk petroleum carrier ships must have separate tanks for ballast and cargo oil. Ships thereupon necessarily became larger in overall size for carrying the same amount of cargo. Fewer bulk petroleum carrier ships were built to this requirement than had been built to serve the same market within a comparable prior period. Also, new and aggressively expanding factors in the bulk cargo vessel field sought to capture market share by cutting out what they deemed excess weight in the construction of hulls for such vessels. Part of the reduction was accomplished by using high tensile strength steel, but some was accomplished by reducing the safety margin in the thickness, spacing and redundancy of constructional elements conventionally provided to accommodate loss of strength due to corrosion occurring during the expected life of the vessel. At the same time, carrying only ballast in certain tanks of the vessel, due to requirements of the MARPOL Convention, caused accelerated corrosion. The need for better coatings was not recognized soon enough; therefore, it is now believed that many bulk cargo-carrying ships built within the last 15 years will have unpredictably short useful lives.
A conventional double-hull tanker lets ballast be carried between hulls. Such a ship does not need to be any larger, overall, than a conventional single-hull, segregated-ballast tanker.
It is conventional to blast-clean and coat the surfaces of ballast tanks upon construction, and thereafter, once each 5 years. Currently, sand or mineral abrasive is used for the abrasive in blast cleaning the tank surfaces, and solvent-based paints are used for originally coating, and recoating them.
Although several coatings manufacturers are believed to be experimenting with water-based, marine paints, it is problematical whether and when such paints will become commercially available and displace solvent-based paints from the marketplace.
Workers at shipyards are becoming less willing to work in a confined space with sand-blasting equipment and with apparatus for applying solvent-based paints.
Coating a vessel results in the generation of refuse, such as empty paint drums, which often must be disposed of as hazardous waste. A painting system which uses less paint (e.g., only about 20 percent as much paint as conventional), potentially reduces the bulk of refuse needing to be disposed of as hazardous waste.
Recent changes to air quality legislation (the Clean Air Act), give certain industrial users no more than 10 years to solve problems of air quality resulting from paint overspray, and release of volatile organic compounds into the air, as solvents evaporate from coatings.
Accordingly, there is a present need in the marketplace, both from an environmental standpoint and from a work force contentedness standpoint of a painting system which provides a longer lasting coating in the specific environment.
Good coating requires cleaning of the surface that is to be coated immediately prior to applying the coating thereto. However, if a coating is applied to components early in a fabrication process, portions of it may be destroyed later by welding and cutting of structure and temporary fitting and holding devices used in the course of constructing a hull.
There is a trend in industrial coatings, particularly in consumer products, towards greater use of cathodic application of epoxy material. Certain cathodic epoxy paints are known to have the capability of providing good protection against both oil and salt water. A conventional marine coating typically survives 400 to 500 hours in a salt spray (accelerated corrosion) test, whereas cathodic epoxy coatings, particularly which recently have become commercially available from PPG Coatings, can survive 2000 hours in the salt spray test. In applying such a coating, the part is shot-blasted, then cleaned, finished and provided corrosion protection through a chemical wash and pretreatment process of three stages or more, and then dipped into a bath of paint particles and water, with the part serving as a cathode causing the paint particles to "plate" onto its surfaces. The coated part is then dried using infrared energy sources at approximately 350.degree. F. Usually, this dip-and-dry process is repeated one or more times, after which the coated part is inspected for integrity of the coating. Parts which pass are considered completed; those which do not, are recycled for remedial coating. This process is believed to be presently in commercial use for coating outboard motors for boats.
Conventionally, fuel oil for powering hhe engines which propel a tanker is stored in deep tanks at the ends of the vessel. In these locations, the vessel fuel oil sometimes is located where it could easily spill, were the vessel hull to become ruptured in an accident. Accordingly, there is a need for a hull construction system which potentially can provide for storage of vessel fuel oil within closed bays of a double deck, remote from the danger of tank rupture and oil spillage.
It is believed that in the period from 1990 to 2010, the number of tankers requiring replacement or remidbodying, assuming modest expansion of world fleet requirements, an average vessel life of 25 years, and an average vessel size of 85,000 DWT, is about 180 to 200 tankers per year.